Download What is Cage Culture? - Alabama Cooperative Extension System

SRAC Publication No. 160
July 2008
Revision
PR
VI
What is Cage Culture?
Michael Masser1
Fish can be cultured in one of four
culture systems—ponds, raceways,
recirculating systems or cages. A cage
or net pen is a system that confines
the fish or shellfish in a mesh enclosure. By strict definition, a cage
and a net pen differ based on their
construction. A cage has a completely
rigid frame (on all sides) and a net
pen has a rigid frame only around
the top. However, the terms “cage”
and “net pen” are often used interchangeably. Marine cages are often
called net pens, even though they
have completely rigid frames, and
vice versa. Structural differences have
little effect on production practices
or the environmental impact of these
systems. This fact sheet will use the
term “cage” collectively. Cage culture
uses existing water resources (ponds,
rivers, estuaries, open ocean, etc.) but
confines the fish inside some type of
mesh enclosure. The mesh retains the
fish, making it easier to feed, observe
and harvest them. The mesh also
allows the water to pass freely between the fish and surrounding water
resource, thus maintaining good
water quality and removing wastes.
Cage culture probably originated with
fishermen who used cages to accumulate fish for market. Over time, they
learned to feed the fish in these cages
to increase their size and improve
their overall health. The first cages
used for culturing rather than just
holding fish were probably developed
in Southeast Asia about the end of
the 18th century. These cages were
Texas AgriLife Extension Service, College Station, Texas.
1
constructed of wood or bamboo and
the confined fish were fed trash fish
and food scraps.
Modern cage culture in the U.S.
began in the 1950s with the advent of
synthetic materials suitable for cage
construction. Universities in the U.S.
began conducting cage culture research in the 1960s. Freshwater cage
research in the U.S. has been limited because large-scale, open-pond
culture was more economically viable
and received most of the research
focus. There has been little research
on marine cage systems because of
regulatory issues, a limited number of
good quality sites, and the high cost
of research. Currently, however, there
is renewed interest in marine cage or
net pen culture in the U.S. Federal
and state regulations are being reexamined in an effort to streamline
permitting processes and define rules,
oversight and liabilities. These issues
have not been resolved at the time of
this writing. Therefore, this series of
fact sheets will be devoted to freshwater cage culture and not address unresolved marine cage culture issues.
Today, freshwater cage culture is practiced to a limited extent in the southern
U.S., usually as an alternative type of
agriculture for small and limitedresource farmers. Freshwater cage culture also allows farmers to use existing
water resources that may or may not
be used for other purposes. The fish
produced are usually sold to local niche
markets. As wild-capture fisheries have
declined and aquaculture has expanded, these niche markets have also
grown. The result may be opportunities
to increase cage production.
Freshwater cage culture is not foolproof or simple. On the contrary, it
is more intensive in many ways than
pond culture and probably should be
considered as an alternative commercial enterprise only where open-pond
culture is not practical because of
excessive depth, obstructions that prevent harvest, or predator problems.
Advantages of Cage Culture
Resource use flexibility
Cage culture can be established in
any suitable body of water, including
lakes, ponds, mining pits, streams
or rivers with proper water quality,
access and legal authority. This flexibility makes it possible to exploit underused water resources to produce
fish. (Specific state laws may restrict
the use of public waters for private
fish production. Contact your state
Extension service or natural resource
agency for regulatory information.)
Low initial investment
Relative to the cost of pond construction and its associated infrastructure
(electricity, roads, water wells, etc.),
cage culture in an existing body of
water can be inexpensive. At low
densities (relative to pond surface
acreage) cages often do not require
aeration or any electrical source.
Cage materials are not especially expensive and many kinds of cages can
be constructed with little experience.
(See SRAC #162, Cage Culture: Cage
Construction and Placement.)
Simplified cultural practices
Cages lend themselves to straightforward observation of the fish. The
observation of fish behavior, especially feeding behavior, is critical to
anticipating and avoiding problems
with stress and diseases, which often
occur in cage culture.
Simplified harvesting
Cages are usually harvested by moving them into shallow water, crowding the fish into a restricted area, and
simply dipping the fish out of the
cage. Or, the cage can be lifted partially out of the water so that the fish
are crowded into a smaller volume,
and then the fish dipped out. This
makes it possible to partially harvest
fish from cages as needed for local
niche markets or personal consumption.
Multi-use of water resources
The confinement of fish in cages
should not hinder other uses of the
water resource, such as fishing, boating, swimming, irrigation or livestock
watering.
These advantages are appealing, particularly the low capital investment
required. A farmer could try producing
fish in an existing pond or other water
resource with minimal financial or
environmental risk. If successful, the
farmer could expand production with
additional cages or intensify production
by increasing aeration or fish densities.
Disadvantages
of Cage Culture
Complete diets needed
Feed must be nutritionally complete
and kept fresh. Caged fish will get no
natural food and so depend on the
manufactured diet for all essential nutrition. Feed must provide all necessary proteins (down to specific amino
acids), carbohydrates, fats (including
essentially fatty acids), vitamins
and minerals for maximum growth.
Nutrients start to deteriorate quickly
when exposed to heat and moisture.
The shelf life of feeds is less than 3
months from the date of manufacture.
Therefore, foods must be stored properly and fed quickly.
Water quality problems
Localized water quality problems,
particularly low dissolved oxygen, are
common in cage culture. The high fish
densities, along with the high feeding
rates, often reduce dissolved oxygen
and increase ammonia concentration
in and around the cage, especially if
there is no water movement through
the cage. Low dissolved oxygen within
cages may not affect other organisms
in the lake, pond or stream.
Diseases
Diseases are a common problem
in cage culture and they can cause
catastrophic losses. Wild fish around
the cage can transmit diseases to the
caged fish. The crowding in cages
promotes stress and allows disease
organisms to spread rapidly.
Vandalism and poaching
Caged fish are an easy target for
poachers and vandals. Cages must be
placed where access can be controlled
and poaching risks reduced.
Predation
Predation can be a problem if cages are
not constructed or managed properly.
Turtles, snakes, otters, raccoons and
fish-eating birds will take fish or damage cages unless precautions are taken.
Limited production
Fewer pounds of fish per acre are produced in cages than in open pond culture systems. This is mostly because
of the stress associated with fish
density, the poorer localized water
quality, and the increased incidence
of disease.
Overwintering problems
It is difficult to overwinter warmwater fish in cages. There is usually a
high mortality rate because of bacterial and fungal diseases.
The disadvantages of cage culture
should be considered carefully before
production begins. Many of these problems can be dealt with through proper
construction, location and management.
Species selection
Research in the southern U.S. has centered primarily on the cage culture of
channel catfish and rainbow trout, with
some studies conducted on carp, hybrid
striped bass, tilapia and sunfish. For a
species to be practical for cage culture,
we must know its culture requirements
and be able to satisfy them. That means
supplying the water quality it needs and
having access to commercial feeds that
meet its nutritional requirements. Some
species, like blue catfish, do not do well
in cages because they cannot tolerate
the crowding that is necessary for efficient cage production.
Before attempting to raise fish in cages,
the producer should carefully analyze
potential markets, the production site,
water quality, construction and production costs, and legal requirements. Of
these, the market is most often overlooked by novice fish producers. Research the markets in your area before
deciding whether or not to culture fish.
Finally, check with your local Extension
office or state department of natural
resources about the permits needed for
aquaculture and the processing and
transportation of fish.
Other publications in this series on cage
culture are:
SRAC #161, Cage Culture: Site Selection
and Water Quality
SRAC #162, Cage Culture: Cage Construction and Placement
SRAC #163, Cage Culture: Species Suitable for Cage Culture
SRAC #164, Cage Culture: Handling and
Feeding Caged Fish
SRAC #165, Cage Culture: Cage Culture
Problems
SRAC #166, Cage Culture: Harvesting
and Economics
SRAC fact sheets are reviewed annually by the Publications, Videos and Computer Software Steering Committee. Fact sheets are revised
as new knowledge becomes available. Fact sheets that have not been revised are considered to reflect the current state of knowledge.
The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No.
2006-38500-16977 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service.